The Coldest Decade in the Past 500 Years
A team of chemists from the U.S. and France has found compelling evidence of a previously
undocumented large volcanic eruption that occurred exactly 200 years ago, in 1809.
The discovery, published in the scientific journal Geophysical Research Letters in October, 2009,
offers an explanation as to why the decade from 1810 to 1819 is regarded by scientists as the
coldest on record for the past 500 years.
Evidence Found in Greenland and Antarctica
"We've never seen any evidence of this eruption in Greenland that corresponds to a simultaneous
explosion recorded in Antarctica before in the glacial record," said Mark Thiemens, Dean of the
Division of Physical Sciences at UC San Diego and one of the co-authors of the study. "But if
you look at the size of the signal we found in the ice cores, it had to be huge. It was bigger
than the 1991 eruption of Mount Pinatubo in the Philippines, which killed hundreds of people
and affected climate around the world."
Led by a chemist from South Dakota State University, the team of scientists made its discovery
after analyzing chemicals in ice samples from Antarctica and Greenland in the Arctic, where the
scientists visited and drilled ice cores three years ago. The year-by-year accumulation of snow
in the polar ice sheets records what is going on in the atmosphere.
Sulfuric Acid Evidence
"We found large amount of volcanic sulfuric acid in the snow layers of 1809 and 1810 in both
Greenland and Antarctica," said professor Jihong Cole-Dai of SDSU's Department of Chemistry
and Biochemistry, who was the lead author of the paper.
Joël Savarino of the Laboratoire de Glaciologie et Géophysique de l'Environment in Grenoble, France,
and a former postdoctoral fellow at UC San Diego, was also part of the team.
Cole-Dai said climate records show that not only were 1816 - the so-called "year without a summer"- and
the following years very cold, the entire decade from 1810 to 1819 was probably the coldest for at least the past 500 years.
Six Years Before Tambora Erupted
Scientists have long been aware that the massive and violent eruption in 1815 of an Indonesian volcano
called Tambora, which killed more than 88,000 people in Indonesia, had caused the worldwide cold weather
in 1816 and after. Volcanic eruptions have a cooling effect on the planet because they release sulfur
gases into the atmosphere that form sulfuric acid aerosols that block sunlight. But the cold temperatures
in the early part of the decade, before that eruption, suggest Tambora alone could not have caused the
climatic changes of the decade.
"Our new evidence is that the volcanic sulfuric acid came down at the opposite poles at precisely the same
time, and this means that the sulfate is from a single large eruption of a volcano in 1809," Cole-Dai said.
"The Tambora eruption and the undocumented 1809 eruption are together responsible for the unusually cold decade."
Probable Tropical Source
Cole-Dai said the Tambora eruption was immense, sending about 100 million tons of sulfur gas into the
atmosphere, but the ice core samples suggests the 1809 eruption was also very large - perhaps half the size
of Tambora - and would also have cooled the earth for a few years. The researchers reason that, because the
sulfuric acid is found in the ice from both polar regions, the eruption probably occurred in the tropics, as
Tambora did, where wind patterns could carry volcanic material to the entire world, including both poles.
Sulfur Isotope Signatures
Cole-Dai said the research specifically looked for and found a special indicator of sulfuric acid produced
from the volcanic sulfur gas in the stratosphere.
The special indicator is an unusual make-up of sulfur isotopes in the volcanic sulfuric acid. Isotopes are
different types of atoms of the same chemical element, each having a different number of neutrons (but the
same number of protons). The unique sulfur isotope composition is like a fingerprint of volcanic material
that has reached the stratosphere, said Cole-Dai.
The stratosphere is the second major layer of the Earth's atmosphere, reaching from about six miles to about
30 miles above the Earth's surface at moderate latitudes. To impact global climate, rather than local weather,
the sulfur gas of a volcanic eruption has to reach up into the stratosphere and once there, be spread around the globe.
A Natural Global-Cooling Experiment
"The way in which that these volcanoes affected the average temperatures of our planet gives us a better idea
of how particulates in the atmosphere can affect our climate," said Thiemens. "People talk about the possibility
of geo-engineering our climate, but the question is how? In this case, nature has done an experiment for us."
Other members of the research team were South Dakota State post-doctoral researcher David Ferris and graduate
student Alyson Lanciki; and Mélanie Baroni of CEREGE (Le Centre Européen de Recherche et d'Enseignement des
Géosciences de l'Environnement) at L'Université Paul Cézanne in Aix-en-Provence, France.
The researchers were funded by the National Science Foundation, French Polar Institute (IPEV) and the Institut
National des Sciences del'Univers (INSU).
Kim McDonald, UCSD, 858-534-7572 or firstname.lastname@example.org
Lance Nixon, South Dakota State, 605-696-7869 or email@example.com
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|The team drilled ice cores in Antarctica and in the Greenland ice sheet. Image by Mark Thiemens, UCSD.
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